Fuel cells are electrochemical devices in which the energy from a chemical reaction is converted to direct current electricity. During operation of a fuel cell, a continuous flow of fuel, e.g., hydrogen (or a liquid fuel such as methanol), is fed to the anode while, simultaneously, a continuous flow of an oxidant, e.g., air, is fed to the cathode. The fuel is oxidized at the anode causing a release of electrons through the agency of a catalyst. These electrons are then conducted through an external load to the cathode, where the oxidant is reduced and the electrons are consumed, again through the agency of a catalyst. The constant flow of electrons from the anode to the cathode constitutes an electrical current which can be made to do useful work.
The Polymer Electrolyte Membrane fuel cell (PEMFC) is one type of fuel cell likely to find wide application as a more efficient and lower emission power generation technology in a range of markets including stationary and portable power devices and as an alternative to the internal combustion engine in transportation. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. They need only hydrogen, oxygen from the air to operate and do not require corrosive fluids like some fuel cells. They are typically fueled with hydrogen supplied from storage tanks or onboard reformers.
The Direct Methanol Fuel Cell (DMFC) is similar to the PEMFC in that the electrolyte is a polymer and the charge carrier is the hydrogen ion (proton). However, liquid methanol (CH3OH) is oxidized in the presence of water at the anode generating CO2, hydrogen ions and the electrons that travel through the external circuit as the electric output of the fuel cell. The hydrogen ions travel through the electrolyte and react with oxygen from the air and the electrons from the external circuit to form water at the cathode completing the circuit.
In the PEMFC and DMFC the combined laminate structure formed from the membrane and the two electrodes is known as a membrane electrode assembly (MEA). The MEA will typically comprise several layers, but can in general be considered, at its basic level, to have five layers, which are defined principally by their function. On either side of the membrane an anode and cathode electrocatalyst are incorporated to increase the rates of the desired electrode reactions. In contact with the electrocatalyst-containing layers, on the opposite face to that in contact with the membrane, are anode and cathode gas diffusion substrate layers.
The anode gas diffusion substrate is designed to be porous and to allow the reactant hydrogen or methanol to enter from the face of the substrate exposed to the reactant fuel supply, and then to diffuse through the thickness of the substrate to the layer which contains the electrocatalyst, usually platinum or platinum-ruthenium metal based, to maximize the electrochemical oxidation of hydrogen or methanol. The anode electrocatalyst layer is also designed to comprise some level of the proton-conducting electrolyte in contact with the same electrocatalyst reaction sites. With acidic electrolyte types protons are produced as the product of the reaction occurring at the anode and these can then be efficiently transported from the anode reaction sites through the electrolyte to the cathode layers.
The cathode gas diffusion substrate is also designed to be porous and to allow oxygen or air to enter the substrate and diffuse through to the electrocatalyst layer reaction sites. The cathode electrocatalyst combines the protons with oxygen to produce water and is also designed to comprise some level of the proton-conducting electrolyte in contact with the same electrocatalyst reaction sites. Product water then has to diffuse out of the cathode structure. The structure of the cathode has to be designed such that it enables the efficient removal of the product water.
The complete MEA can be constructed by several methods. The electrocatalyst layers can be bonded to one surface of the gas diffusion substrates to form what is known as a gas diffusion electrode. The MEA is then formed by combining two gas diffusion electrodes with the solid proton-conducting membrane. Alternatively, the MEA may be formed from two porous gas diffusion substrates between which is sandwiched a solid proton-conducting polymer membrane having electrocatalyst layers on both sides (also referred to as a catalyst coated membrane or CCM); or indeed the MEA may be formed from one gas diffusion electrode, one liquid diffusion substrate and a solid proton-conducting polymer having an electrocatalyst layer on the side facing the gas/liquid diffusion substrate.
Although the theory behind fuel cell operation has been known for many years, there has been difficulty producing commercially viable fuel cells due to technological barriers, and also due to the availability of more cost-effective energy sources such as petroleum. However, devices using petroleum products, such as the automobile, produce significant pollution and may eventually become obsolete with the depletion of petroleum resources. As a result, there is a need for an alternative means for producing energy. Fuel cells are a promising alternative source of energy in that they are relatively pollution-free and utilize hydrogen, a seemingly infinite fuel source.
Among the critical issues that must be addressed for the successful commercialization of fuel cells is developing MEAs exhibiting the highest possible performance expressed as power density per unit area (mW/cm2) at certain operating voltage—typically 0.4 to 0.55 V for the DMFC system. Producing MEAs with high absolute performance is highly desirable because it allows the manufacture of smaller, lighter, longer running and more efficient DMFC-based power sources. Cost and durability are the other two major requirements of the DMFC MEAs.
There are several key elements in ensuring high performing MEA in DMFC configuration—electrocatalyst, printed layers and MEA structures, membrane and gas diffusion electrodes. Of these, the electrocatalyst is the most significant performance and cost factor. Pt and PtRu blacks are the electrocatalysts widely used for achieving high power densities, however they suffer from inherently low utilization when printed in electrode layers. Moreover, they lack the requisite durability and are too expensive for commercial viability. Thus, in more recent applications, the electrocatalytic material, particularly Pt and PtRu, is dispersed as nanoparticles on a particulate support material, such as a carbon black or metal oxide.
The motivation for developing supported catalysts is the potential for high precious metal utilization, which becomes especially important when the DMFC devices are targeted for mass market introduction. Achieving high utilization of the expensive precious metal catalysts is highly desirable since it has impact on both performance and the cost. Ability to achieve highest performance value expressed by highest power with lowest amount of precious metal (mW/mgPt) ensures DMFC devices can be cost competitive with the existing power sources and be successfully commercialized. Another critical factor in meeting the commercialization goals for DMFC is meeting durability targets, which are typically several thousand hours. Supported electrocatalysts typically exhibit improved durability as compared with metal blacks.
The typical production method for carbon-supported Pt and PtRu electrocatalysts is a batch process, which starts by precipitating or impregnating Pt metal precursors followed by chemical reduction techniques in slurry of carbon black. Where necessary, the addition/alloying of the second metal (Ru) is achieved by precipitation of the second precursor onto the Pt clusters and the alloying of the PtRu is achieved through reduction in hydrogen at high temperature (see, for example, U.S. Pat. No. 6,326,098). Another approach (see, for example, U.S. Pat. No. 5,068,161 and “Preparation of Highly Dispersed Pt+Ru Alloy Clusters and the Activity for the Electrooxidation of Methanol” by Watanabe et al., J. Electroanal. Chem., 229 (1987), pages 395-406) relies on colloidal precipitation and deposition of the PtRu colloidal particles onto the carbon support. All of these synthesis methods very often lead to the formation of PtRu clusters which do not have simultaneously high dispersion, high degree of crystallinity and high stability (or durability when exposed to acidic environment during the operation of DMFC). The poor stability of PtRu alloys and the dissolution of Ru during long-term operation of the DMFC is a major contributing factor for the loss of initial performance and poor long-term durability. The ability to generate highly performing and durable PtR catalysts with a high degree of normalized performance (mW/mgPt) and utilization is necessary for commercialization of direct methanol fuel cells.
More recently, a highly reproducible, low cost, continuous powder manufacturing process based on spray conversion has been developed, which is capable of achieving excellent control over the dispersion, composition and microstructure of electrocatalyst compositions leading to unrivaled electrochemical performance. In the spray conversion process, a liquid-containing feedstock comprising dissolved non-volatile electrocatalyst precursors and suspended solids is atomized to form droplets and the droplets are heated to form powders. This process offers the advantage of producing electrocatalyst powders with unique morphology comprising a combination of highly active and dispersed nanoparticles on a mesoporous carbon support with a micron size aggregate structure. It is believed that this combination offers the most advantageous layer structure when the catalysts are printed onto a polymer electrolyte membrane. In addition when PtRu alloy-based supported catalysts are produced by the spray based technology, simultaneously high PtRu crystallite dispersion can be achieved, combined with high degree of alloying and extended durability when utilized in fuel cell operating conditions.
A representative example of the spray conversion process can be found in U.S. Patent Application Publication No. 2004/0072683, which discloses a process for making an electrocatalyst powder batch, comprising the steps of: a) providing a liquid-containing precursor composition to said electrocatalyst powder; b) atomizing said liquid-containing precursor into precursor droplets; c) heating said precursor droplets to a reaction temperature of not greater than about 700° C. to form electrocatalyst particles; and d) collecting said electrocatalyst particles. In one embodiment, the electrocatalyst particles are composite electrocatalyst particles comprising a support phase, such as particulate carbon, and an active species phase, such as platinum and/or ruthenium, dispersed on the support phase. When the active species phase is a metal or metal alloy, additives capable of reducing the metal precursor(s) to the metal/alloy at the temperatures employed in step (c) are included in the liquid-containing precursor composition so that the desired reduction is achieved during the spray conversion process.
According to the present invention, it has now been found that when spray conversion is used to produce an electrocatalyst composition, the reduction and, where applicable, the alloying step can be effected as a separate low temperature (no greater than 250° C.) post treatment step in a reducing atmosphere, rather than by adding a reducing agent to the liquid-containing precursor composition such that the reduction occurs during the spray conversion process. Surprisingly, it is found that the product of such a low temperature post treatment step is a novel electrocatalyst composition having enhanced surface area and specific activity.